There was a very advanced civilization who were able to build or reconstruct worlds.They could take a dead planet or moon which would be flying in habitable zone and reconstruct it into world capable to sustain life.If there was no planet in the right distance from the star they used recourses from other planets-moons to build one. After unknown reasons this civilization went extinct many millions of years ago. But what was left is thousands of artificial worlds thriving with life....

Or something like that I don't know i kind of disappointed in this piece. Maybe i will paint another one... maybe...